Staging: add driver for Realtek RTS5139 cardreader

This driver is used for Realtek RTS5139 USB cardreader, which
supports many cards, such as SD, MS, XD series cards.

Signed-off-by: edwin_rong <edwin_rong@realsil.com.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>